Thermal-type flow-rate sensors such as those described in Japanese Unexamined Patent Application Publication No. 2010-133897 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2013-003068 (Patent Document 2) are known.
A plan view of a diaphragm structure, which is an important part of the thermal-type flow-rate sensor described in Patent Document 1, and a region in the vicinity thereof are illustrated in FIG. 13. A sectional view taken along the line XIV-XIV in FIG. 13 and looking in the direction of the arrows is illustrated in FIG. 14. This thermal-type flow-rate sensor is for measuring the flow rate of a flow of a gas in the flow direction 4. A substrate opening 1012 is provided so as to penetrate through a substrate 1001. A protective film opening 1011 is provided in a protective film that covers an upper surface of the substrate 1001. A heating resistor 1003 is arranged in the center of the diaphragm structure, which is provided so as to close off the substrate opening 1012. An upstream temperature-measurement resistor 1005 and a downstream temperature measurement resistor 1006 are arranged on opposite sides of the heating resistor 1003 relative to the flow direction 4.
A plan view of the thermal-type flow-rate sensor described in Patent Document 2 is illustrated in FIG. 15. A sectional view taken along line XVI-XVI in FIG. 15 and looking in the direction of the arrows is illustrated in FIG. 16. This thermal-type flow-rate sensor is for measuring the flow rate of a flow of a gas along the flow direction 4. A cavity 2013 is provided in a substrate 2001 so as to be recessed downward from one surface of the substrate 2001. A heating resistive element 2003, a plate-shaped portion of which is arranged so as to cover the upper side of the cavity 2013, is arranged in the center of the cavity 2013. An upstream temperature-measurement resistive element 2005 and a downstream temperature-measurement resistive element 2006 are arranged in the flow direction 4 with the heating resistive element 2003 interposed therebetween.
As is clear from Patent Documents 1 and 2, a thermal-type flow-rate sensor according to the prior art includes a substrate having an opening therein, an insulating layer provided to cover the opening, a heating element, an upstream temperature measurement element and a downstream temperature measurement element. The heating element is provided on part of the insulating layer that is arranged over the opening in the substrate. The upstream temperature measurement element is provided on part of the insulating layer that is arranged over the opening in the substrate and that is upstream of the heating element in the direction in which the gas (fluid) flows. The downstream temperature measurement element is provided on part of the insulating layer that is arranged above the opening in the substrate and that is downstream of the heating element in the direction in which the gas (fluid) flows. The upstream temperature measurement element and the downstream temperature measurement element are arranged at equal distances from the heating element. The upstream temperature measurement element and the downstream temperature measurement element are composed of the same material as the heating element.
A thermal-type flow-rate sensor according to the prior art causes the heating element to generate heat by supplying an electrical current to the heating element. An electrical current is also supplied to the upstream and downstream temperature measurement elements. When the gas (fluid) is not flowing, the upstream temperature measurement element and the downstream temperature measurement element, which are at equal distances from the heating element, have substantially the same temperatures and their resistance values are the same. On the other hand, when the gas (fluid) flows, the heat generated by the heating element is transmitted to the downstream temperature measurement element via the gas flow and the temperature of the downstream temperature measurement element rises. Consequently, a temperature difference is generated between the upstream temperature measurement element and the downstream temperature measurement element and the resistance value of the upstream temperature measurement element and the resistance value of the downstream temperature measurement element are different. The actual temperature difference can be derived from the difference between the resistance value of the upstream and downstream temperature measurement elements. Since the temperature difference between the upstream and downstream temperature measurement elements is proportional to the square root of the flow speed of the gas, the flow speed of the gas can be obtained from the derived temperature difference.
In the foregoing structures, current needs to be continually supplied to the heating element, the upstream temperature measurement element and the downstream temperature measurement element. Consequently, there is a problem in that power consumption is large.
Accordingly, an object of the present invention is to provide a thermal-type flow-rate sensor that can reduce power consumption.
In order to achieve this object, a thermal-type flow-rate sensor according to one embodiment of the present invention includes a substrate and a detection unit that is supported by the substrate. At least an upper surface of the detection unit is exposed to a flow of a fluid in a flow direction. The detection unit includes an insulating layer, a heating element arranged on an upper surface of the insulating layer, an upstream temperature measurement element arranged on the upper surface of the insulating layer upstream of the heating element in the flow direction, and a downstream temperature measurement element arranged on the upper surface of the insulating layer downstream of the heating element in the flow direction. Both temperature measurement elements include a pyroelectric layer that do not require a current to flow through them to work.
According to the present invention, a thermal-type flow-rate sensor can be realized that can reduce power consumption.